A Harvard University police officer on a routine patrol is believed to be responsible for catching the arsonist who set a house fire that left a fire-fighter hospitalized Saturday.

Officer John Okane, who was patrolling near the Harvard Medical School, observed a man who appeared agitated and was trying to interfere with firefighters who had been called to the blaze at 46 Fen-wood Road.

As a result of the Harvard officer's observations, State Police arrested Michael A. Blackwell, 39, of Boston. Blackwell was scheduled to be arraigned today on a charge of arson of a dwelling.

Police said Blackwell poured gasoline on the back of the triple-decker in the Mission Hill district, then stuck around to watch it burn.

Twelve people were left homeless by the fire. Six had to be rescued by firefighters, three of them with ladders.

"It was very surreal--big balls of fire, people hanging out the windows, lots of smoke," said Nick Valentino, a resident. "It was very strange."

Fire Capt. Robert Calobrisi was knocked unconscious while helping to rescue tenants. He was in serious condition Saturday at Brigham & Women's Hospital.

The fire was reported at 5 a.m.

According to a Harvard police report, Officer Okane spotted Black-well after he was called to the scene to assist with crowd control.

"[Blackwell's] movements were highly agitated and he was actually bouncing up and down and shouting," according to the police report.

"At this point he was asked to leave and had to be physically removed, to which he became extremely angry," the report continues. "[He] became extremely belligerent and began to shout racial epitaphs."

This story was compiled with Associated Press wire dispatches.